In a situation where higher requirements in terms of operational, fire, and contact safety have to be met when supplying electrical equipment with energy, the network type of an ungrounded power supply system is used, which is also known as an insulated network (French: isolé terre—IT) or IT system. In this kind of power supply system, the active parts are separated from the ground potential, i.e. against ground. The advantage of these networks is that the function of the electrical equipment is not affected in case of a first insulation fault, such as a fault to ground or a fault to frame, because the ideally infinitely large impedance value prevents a closed circuit from forming between the active conductors of the network and ground in this first fault case (first fault).
In a three-phase IT system, the outer conductors L1, L2, L3 and, if present, the neutral conductor N are referred to as the active conductors. In a single-phase IT system without a center tap, the two outer conductors L1 and L2 are the active conductors; in case of a center tap, the mid-point conductor is an active conductor, too.
The inherent safety of the IT power supply system thus ensures a continuous power supply of the loads fed by the IT power supply system, namely of the equipment connected to the IT power supply system, even if a first insulation fault occurs.
Hence, the resistance of the IT power supply system against ground (insulation resistance; also called insulation fault resistance or fault resistance in the fault case) is continuously monitored because another potential fault on another active conductor (second fault) would cause a fault loop and the resulting fault current, in connection with an overcurrent protection device, would lead to a shutdown of the installation and to a standstill of operation.
Based on the condition that the state of insulation of the IT power supply system is continuously monitored by an insulation monitoring device, the IT power supply system can continue to operate without a prescribed time limit even when a first fault has occurred. Still, it is recommended according to standards DIN VDE 0100-410 and IEC 60364-4-41 that the first fault be eliminated as quickly as practically possible.
To meet the requirement of quick elimination of the first fault, the use of an insulation-fault search system is the state of the art especially in extensive, complex IT power supply systems or in IT power supply systems in which a shutdown of the power supply for the entire IT power supply system may be safety-critical.
If a first insulation fault has been recognized in the IT power supply system by the insulation monitoring device, the insulation-fault search begins in that a test-current generator, which may be a separate device or part of the insulation monitoring device, generates a test current and feeds it into the IT power supply system at a central point. This test-current signal is registered (test-current registering device) by all measuring-current transformers located within the faulty line outlet of a subsystem and is evaluated and displayed by an insulation-fault evaluating device. By associating the measuring-current transformer with the line outlet (subsystem), the fault location can be located.
Despite these advantages, the IT power supply system is still used relatively rarely compared to grounded power supply systems. One reason for this are the three problematic areas mentioned below.
A first problem (excess voltage) relates to the danger that when a first insulation fault occurs in an IT power supply system in one of the outer conductors, a potential difference against the ground potential occurs on the other active conductors for which the connected equipment (loads) is not designed. This excess voltage is problematic if the suppression measures (e.g. Y capacitors) on the connected equipment are not designed for this increased potential difference. Moreover, the insulating materials of the lines are under increased strain as well and may sustain permanent damage.
This problem occurs in particular if connected equipment originally designed for the use in grounded power supply systems is used in IT power supply systems in unchanged form and without review of the equipment properties.
Oftentimes, the only solution common thus far to this problem is the time-consuming and costly replacement of the unsuitable equipment with equipment having suitable properties. Alternatively, a complex redundant reconfiguration of the grounded power supply system is necessary in order to achieve the high availability and thus reliability of supply of an IT power supply system.
A second problematic area (displacement direct voltage) relates to applications in which IT alternating-current (AC) supply systems having coupled direct-current (DC) branches, such as a DC intermediate circuit in controlled drives, are combined with equipment that, in case of an occurring displacement direct voltage, which is generated by a poor insulation resistance of the DC circuit and exceeds a limit tolerated by the equipment, can be brought into a critical operating state, which may lead to failure of the equipment.
Monitoring of the DC displacement voltage by means of a voltage relay in connection with a shutdown of the controlled drive causing the DC displacement voltage because of an insulation fault in the DC intermediate circuit, for example, is known from the state of the art. In case of multiple potential sources of DC displacement voltages, however, determining the system branch to be shut down is difficult or even impossible.
Alternatively, the use of a B-type residual current protection device (RCD) is known for shutting down the system branch in which the DC residual current is flowing. However, this measure is accompanied by the disadvantage that in case of high-powered drives with high load currents, even a small asymmetry or weak saturation effects of the residual-current sensor technology may cause false tripping.
As a third problematic area (subsystem shutdown), it is often necessary in large, often highly branched IT power supply systems to quickly shut down the subsystem only in which a potentially hazardous fault (second fault) has occurred without also shutting down other subsystems or even the entire IT power supply system.
In extensive IT power supply systems according to the state of the art, this problem has been addressed by using directional residual current monitors (RCMs).
For the directional detection to function reliably, the condition that the total value of the network leakage capacitances on the network side (“upstream” of the respective residual current measuring device) is many times larger than the total value of the network leakage capacitances on the load side (“downstream” of the respective residual current measuring device) has to be fulfilled. Product standards prescribe a ratio of at least 6:1.
Meeting this requirement means high technical expenditures for both the installer and the operator of the electrical installation. Practice has shown that fulfillment of this condition necessary for the reliable operation of an electrical protection device cannot be safely ensured under all operating conditions and throughout the entire lifespan of the electrical installation.
Thus, it has to be stated that there are solution approaches to the three mentioned problematic areas, but they only address one problem at each and do not take a higher perspective as a basis to find a joint solution for all three problems. Moreover, the task of insulation-fault search is neglected.